Control system for the circulatory system of a shot blasting apparatus

ABSTRACT

A blasting apparatus for recuperating and cleaning shot which includes a circulatory system for the shot, in which the contaminated shot traverses one or several cleaning installations whereby the cleaned shot is fed to centrifuging wheels while inadequately cleaned shot is caused to traverse once again the cleaning installations; at least one measuring device is provided in the circulatory system for determining the degree of contamination of the shot whereby the measuring device is connected with a control device that so acts by way of an actuating element and a mechanical control element and thereby influences the circulatory system in such a manner that a predetermined degree of cleanliness of the shot is maintained at the inlet of the centrifuging wheels.

United States Patent 1191 Graf et al.

1 1 CONTROL SYSTEM FOR THE CIRCULATORY SYSTEM OF A SHOT BLASTING APPARATUS [75] Inventors: Max E. Graf, Karlsruhe; Helmut Roth, Hohenwettersbach; Jl'irgen W.

Von Fahland, Karlsruhe, all of Germany [73] Assignee: Badische Machinenfabrik GmbH,

Karlsruhe, Germany [22] Filed: July 29, 1971 [21] Appl. No.: 167,219

[30] Foreign Application Priority Data Apr. 7, 1971 Germany P 21 16 942.6

521 u.s.c| .1 ..s1/s,51/9,51/14 511 Int. Cl. B24c 3/14, B240 7/00 [58] Field of Search 51/5, 8, 9, 12,14

[ 56] References Cited UNITED STATES PATENTS 2,686,991 8/1954 Powell 51/14 1 51 July 3, 1973 Primary Examiner-Donald G. Kelly Att0rneyCraig, Antonelli & Hill 5 7 ABSTRACT A blasting apparatus for recuperating and cleaning shot which includes a circulatory system for the shot, in which the contaminated shot traverses one or several cleaning installations whereby the cleaned shot is fed to centrifuging wheels while inadequately cleaned shot is caused to traverse once again the cleaning installations; at least one measuring device is provided in the circulatory system for determining the degree of contamination of the shot whereby the measuring device is connected with a control device that so acts by way of an actuating element and a mechanical control element and thereby influences the circulatory system in such a manner that a predetermined degree of cleanliness of the shot is maintained at the inlet of the centrifuging wheels.

25 Claims, 2 Drawing Figures PAIENIEBJULB ms SIEEIIOFZ JURGEN W.vo FAHLAND BY (1,04 Q wtOQAIQQX 4, Hum

ATTORNEYS PATENTEDJULB ms 3.742.650

SHEEP? 0F 2 lnremars MAX E.GRAF, HELHUT Row M;

IURGEN W.von FAHLAND BY aqu ommw e HM ATTORNEYS CONTROL SYSTEM FOR THE CIRCULATORY SYSTEM OF A SHOT BLASTING APPARATUS The present invention relates to a blasting apparatus with a circulation of the blasting medium, such as shot, in which the contaminated shot is fed by a conveyor mechanism to at least a single-stage cleaning installation whereby the cleaned shot is fed to the centrifuging wheels and the contaminants are separated out of the shot circulatory system, and in which a control section is provided by means of which the shot is conducted either into a supply bin disposed upstream of the centrifuging wheels or into the shot circulatory system of the blasting machine.

Blasting machines for cleaning castings are known already in various types of constructions. The shot is thereby conducted into a circulation from which contaminants are separated with the aid of corresponding separators.

It is additionally known in the prior art to provide shifting valves or flaps in the shot circulatory system upstream or downstream of'the cleaning installation. If such a valve or flap is arranged upstream of the cleaning installation, then an overloading of the cleaning installation in case of an excessive amount of sand accumulation is to be avoided. If the valve is disposed downstream of the cleaning installation, then one intentionally permits an overloading of the cleaning installation and the shot cleaned only inadequately during the duration of the overload is not fed to the centrifuging wheels but, for the most part, is conducted back to the cleaning installation by way of a supply bin whereas the centrifuging wheels are fed with welLcleaned shot from another supply bin.

It is a known fact that the wear, especially in the turbines, is dependent on the degree of contamination of the shot which is fed to the turbines. The efficiency and economy of the operation is therefore also a function of the cleanliness of the shot.

It is known that the sand content of the shot alters very suddently during the rythmic or cyclic machine operation such that a non-uniform accumulation of the sand occurs. The increase in the amount of sand generally exceeds the capacity of the cleaning system for the shot for short periods of time which provides for unfavorable operation. Additionally without regard to the non-uniform changes in sand content, other changes slowly occur in the cleaning system which effect the quality of cleaning, such as, for example, changes in the air quantity for air separation which effects the shot distribution and which results from improper maintenance of filters, air ducts, fans or the like of the blasting apparatus. In particular, the last-mentioned changes take place for the most part slowly and unnoticeably so that they are not readily noticed by the operating personnel.

Insofar as shot samples are examined in the labora-- tory, an accurate control is realized in the time interval of the sample examinations which, however, does not exclude abnormal or faulty conditions in the period of time between the examinations. In most cases, only a subjective manual examination is undertaken by the operating person which, on the one hand, entails the advantage that these examinations can be undertaken in short time intervals, however, on the other hand, the results thereof are extraordinarily unrealiable. Added thereto is the fact that the contamination or soiling can change rapidly in time so that in addition to the other uncertainties, must be added that of the correct timing of the sample removal. One should also further consider that the required tolerance of the cleanliness in the shot of, for example, 0.3 to 0.8 per cent by weight of sand in the shot is not detectable with the human senses.

Accordingly, the aim of the present invention in the realization of a blasting machine with low wear and therefore with an economic operation resides in a control carried out continuously during the entire operating period and with high accuracy of the sand content in the shot and with automatic interaction derived therefrom in the shot circulation with the aim to maintain a predetermined cleanliness degree of the blasting medium fed to the centrifuging wheels without the necessity that the operating personnel has to supervise these operations. The present invention is characterized in that at least one measuring installation for the determination of the degree of the contamination is provided in the shot circulatory system whereby this measuring installation is operatively connected with a control installation which influences the shot circulation by way of a suitable drive and mechanical control part in such a manner that the shot at the inlet of the centrifuging wheels does not fall below a predetermined degree of cleanliness and purity.

One embodiment of the application of the present invention resides in that the measuring installation is arranged as closely as possible downstream of the treating location of the workpiece and the measurement results thereof influence by way of the control installation the drive and mechanical control part in the form of a throttle in such a manner that the shot supplied to the centrifuging wheel is so adjusted that the quantity of blasted-off sand plus shot does not exceed the capacity of the cleaning installation. A simple measuring installation for that purpose consists thereby of two induction coils of which one is traversed by the shot mixture to be measured and the other has a filling that does not change. It is furthermore appropriate, especially if warm workpieces are blasted, that the two coils are kept at a temperature by a heating device which corresponds approximately to the highest shot temperature.

Another example of an application of the present invention resides in that a measuring installation is ar ranged downstream of the cleaning installation and upstream of the centrifuging rotor whose measurement results act by way of the control installation on the drive and the mechanical control part in the form of a shifting valve in such a manner that the well-cleaned shot is fed to the centrifuging wheels and the poorly cleaned shot is conducted back into the shot circulation in by-passing relationship to the centrifuging wheels for renewed cleaning. A measuring installation which is suitable in particular for the determination of slight quantities of contaminants, i.e., for measurements, downstream of the cleaning installation, consists of a source of radioactive rays and of a receiver for the radioactive rays which aredisposed on two sides of the line conducting the shot whereby the ray receiver produces the measurement result which is amplified in an amplifier of conventional construction and changed into a pulse by conventional means.

A prerequisite for completely satisfactory measurement is characterized in that all measurements are undertaken in a vertically arranged line section that is followed by a throttle place. By reason of the dependence of the measurement on the distribution of grain size of the shot, it is appropriate that a metering device meters fresh shot from a supply bin to the shot circulatory system approximately to the same extent as the use thereof.

Accordingly, it is an object of the present invention to provide a blasting apparatus of the type described above which eliminates by simple means the aforementioned shortcomings and drawbacks encountered in the prior art.

Another object of the present invention resides in a blasting apparatus for cleaning work pieces, for example, castings, which excels by improved economy and efficiency of operation.

A further object of the present invention resides in a blasting apparatus of the type described above which automatically controls the cleanliness of the blasting medium fed to the blasting means in order to prevent undue, premature wear thereof.

Still another object of the present invention resides in a blasting machine which is equipped with a completely automatic control system to control fully automatically the degree of cleanliness of the shot circulated in the machine while at the same time precluding overloading of the cleaning installations thereof.

These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing, which shows, for purposes of illustration only, one embodiment in accordance with the present invention, and wherein:

FIG. 1 is a schematic view of a blasting apparatus with measurement locations and control installations in the shot circulatory system in accordance with the present invention; and

FIG. 2 is a partial, somewhat schematic, view of one measurement location provided with induction coils and heating means in accordance with the present invention.

Referring now to the drawing and more particularly to FIG. 1, reference numeral 1 designates in this figure a blasting cabin or chamber within which the workpiece 4 is cleaned by means of shot blasted against the workpiece from conventional centrifuging wheels 2 or the like. An overhead conveyor 3 of conventional construction carries the workpiece 4 into the chamber 1. The collection of a mixture of shot and foreign materials S, for example, quartz, sand and core material of castings, collects in the lower part of the apparatus. A conventional conveyor 6, driven by an electrically controllable drive 7 feeds this mixture into the discharge line 8. A vertically arranged discharge line disposed parallel to the discharge line 8 and provided with an induction coil 9 includes downstream thereof an adjustable throttling place 10 of any conventional, known construction. A compensating coil is designated by reference numeral 11. Both coils have a common energizing line 45 and separate measurement lines 46 and 47. Reference numeral 12 designates a conventional electrical apparatus which contains the measurement and amplifier means and includes control lines 48 and 49 as output means. In the illustrated embodiment, the rotational speed of the drive 7 is adjusted by way of these control lines 48 and 49. However, the throttle 57 may be adjusted in addition thereto or alone by way of the drive 58 by means of the control realized through the lines 59 and 60 indicated in dash lines. Since the control device 12 is of conventional construction utilizing conventional, commercially available electrical com ponents, such as logic circuit elements, a detailed description thereof is dispensed with herein.

A conventional bucket conveyor 13 feeds the mixture in the upward direction and empties the same into a channel 14 for the air separation device 15. A blower (not shown) sucks the air at the connection 16 through the cascades whereby the light weight and fine foreign materials are separated from the heavy and coarsegrained shot. A deflection plate 30 is disposed in the expansion space 17; the coarser parts of the foreign materials carried along by the air drop off in the downward direction by the effect of the deflection plate 30 and are separated out of the circulation by way of the line 18. Cleaned shot flows from the air-sifting device 15 through the line 20 by way of the shifting valve 21 and throttle place 22 either into the funnel 23 and line 29 back into the chamber lower part or through the funnel 24, collecting or storage container 27 and line 28 to the centrifuging wheel 2. The source 19 of radioactive rays irradiates the line 20 and the mixture flowing through the line and acts on the receiver section 25 which is connected by way of the measurement lines 50 and 51 with the evaluation device 37 provided with an amplifier section whose output 52 and 53 shifts the valve 21 by way of the actuation means 26 into the one or other end position depending on the results of the measurements. Since the actuation, for example, an electromagnetic actuation as well as the device 37 including its amplifiers are of conventional construction consisting of commercially available parts, a detailed description thereof is dispensed with herein. A container or bin 54 for new shot has a metering device 55 with its drive or actuating means 56 of any conventional type.

In FIG. 2, reference numeral 31 represents a vertically arranged tube or pipe through which flows a mixture of shot and foreign material; the pipe 31 consists of non-ferromagnetic material with heat-storing properties, for example, of any known plastic or synthetic resinous material. An induction coil 32 is wound upon the pipe 31. A second pipe 33 of the same dimensions and closed by means of plugs 36 is arranged adjacent the pipe 31; the second pipe 33 is filled, for example, with pure, clean shot 35 and is provided with a compensation winding 34 of same construction as the induction coil 32. A common housing 39 of magnetic material, for example, of iron plates contains a heatinsulation 38. An air space 40 is provided about both induction coils 32 and 34 having a supply line 44 for heated air out of the heating body 42, which is supplied by a blower 11 and leaves the coil housing at 43.

For optimum results, the length of the windings 32 and 34 should be at least three times the internal dimension of the pipe diameter while the non-magnetic plastic pipes 31 and 33 should have a wall thickness of about one-third to one-fifth of the inner pipe diameter. Preferably, a heat-insulation layer is applied over the windings 32 and 34.

The operation of a blasting apparatus according to the present invention may take place in such a manner that only a single measuring location is provided or is utilized such as or the measuring location 9 whose measurement results act by way of the control device 12 and the drive 58 on the mechanical control element in the form of a throttle 57 in such a manner that upon indication of a proportion of sand excessive for the air separation installation 15, the quantity of shot for the centrifuging wheel 2 is reduced in such a manner that the quantity of sand blasted off the workpiece 4 is reduced. As soon as the measuring place 9 ascertains that the sand content has become so low that the air separation device is no longer fully loaded, the throttle 57 is reversely opened whereby the blasted off sand quantity again increases. Whereas with this type of the control the over-all quantity of the blasting medium mixture fed to the air separating device 15 remains uninfluenced and only the amount of sand content changes, in case of influencing the conveyor drive 7, the effect is such that the over-all quantity fed to the air separating device 15 changes whereas the sand content remains the same. In both cases, the control is so adjusted that the air separation 15 produces only completely satisfactorily cleaned shot.

Another possibility of the use of the present invention is the application of a measuring place 19, 25 downstream of the air separator 15. The control is thereby so adjusted that when the degree of purity of the shot falls below a predetermined degree, the mechanical control element in the form of the shifting valve 21 is so adjusted by way of the drive 26 that the incompletely or inadequately cleaned shot is fed back for renewed cleaning to the shot circulation under bypassing relationship of the centrifuging wheels whereas the centrifuging wheels are fed from the supply bin 27.

However, also several measurement and control installations may be provided in one and the same blasting apparatus whereby the measurement places 19, 25 may take over the functional control of the measurement place 9 and the control installation thereof.

A particularly economically operating machine with simple constructive realization is obtained by the application of the described features of the present invention, which can be used for an automatic operation without supervision of the individual operating conditions by persons and without control valves actuated manually according to only subjective feels.

While we have shown and described only one embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art. For example, the present invention is not limited to the cabin blasting apparatus of the embodiment shown in the drawing but may be used also with any other type of blasting apparatus, for example, in blasting apparatus with drums or flow operation for continuously or periodic operation with free jet nozzles or centrifuging wheels. Also, the location of the measuring places may depart from the illustrated embodiment. Furthermore, the measuring installation according to the present invention, may also be utilized apart from a machine, for example, for the separate preparation of contaminated shot. With the use of non-magnetic shot, of course, only the irradiation method can be used.

Thus, while we have shown and described only one embodiment in accordance with the present invention, we do not wish to be limited thereto but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

We claim:

l. A blasting apparatus with a circulatory system for shot in which the contaminated shot traverses at least one cleaning means and in which the cleaned shot is fed to a blasting means, characterized in that at least one measuring means is provided in the circulatory system for determining the degree of contamination, said measuring means being operatively connected with a control means controlling the circulatory system by way of an actuating means and a mechanical control element in such a manner that at the inlet of the blasting means the shot does not fall below a predetermined degree of purity.

2. A blasting apparatus according to claim 1, characterized in that the blasting means is formed by centrifuging wheel means.

3. A blasting apparatus according to claim 1, characterized in that means are provided for feeding insufficiently cleaned shot back to the cleaning installation for renewed cleaning action.

4. A blasting apparatus according to claim 1, characterized in that the measuring means is arranged downstream of and close to the treating place of a workpiece and in that the results of the measuring means act by way of the control means on the actuating means and the mechanical control element which is in the form of a throttle means, in such a manner that the shot supplied to the blasting means is so adjusted that the quantity of shot plus sand blasted off from the workpiece does not exceed the capacity of the cleaning means.

5. A blasting apparatus according to claim 1, characterized in that a measuring means is arranged downstream of the cleaning means and upstream of the blasting means, the measuring results of said last-mentioned measuring means acting by way of the control means on the actuating means and mechanical control ele ment which is in the form of a shifting valve means, in such a manner that sufficiently cleaned shot is fed to the blasting means and inadequately fed shot is fed back to the circulatory system for a renewed cleaning operation in by-passing relationship to the blasting means.

6. A blasting apparatus according to claim 1, characterized in that a measuring means which is arranged as near as possible to the treating location of a workpiece acts on the drive means of a conveyor mechanism by way of the control means in such a manner that the sand-shot quantity fed to the cleaning means does not exceed the capacity of the cleaning means.

7. A blasting apparatus according to claim 1, characterized in that the measuring means includes two induction coil means of which one is traversed by the shot mixture to be measured and the other is provided with a non-changing filling, the results of which are amplified and shaped into a control pulse in an amplifier means.

8. A blasting apparatus according to claim 7, characterized in that a heating means is provided for maintaining the two coil means at a predetermined temperature which corresponds approximately to the maximum shot temperature.

9. A blasting apparatus according to claim 8, characterized in that the induction coil means of the measuring means have each a length that is at least three times the inner diameter of a pipe means about which the two induction coil means are wound, said pipe means consisting of non-magnetic material with good heat insulation properties and a wall thickness of about one-third to one fifth of the pipe diameter, and magnetic shield means about the coil means.

10 A blasting apparatus according to claim 9, characterized in that a heat insulation layer is arranged over of radioactive rays and a ray-receiver means which are disposed on two sides of a line means conducting shot, the receiver means producing in its output the measurement result which is amplified and shaped into a pulse in an amplifier means.

12. A blasting apparatus according to claim 11, characterized in that the measuring means are arranged for determining the degree of contamination of the shot in a substantially vertically arranged line section which is followed downstream thereof by a throttle means.

13. A blasting apparatus according to claim 12, characterized in that a metering means is provided for feeding fresh shot from a supply container to the circulatory system approximately to the same extent as use and loss thereof.

14. A blasting apparatus according to claim 4, characterized in that a measuring means is arranged downstream of the cleaning means and upstream of the blasting means, the measuring results of said last-mentioned measuring means acting by way of the control means on the actuating means and mechanical control element which is in the form of a shifting valve means, in such a manner that sufficiently cleaned shot is fed to the blasting means and inadequately fed shot is fed back .to the circulatory system for a renewed cleaning operation in by-passing relationship to the blasting means.

15. A blasting apparatus according to claim 14, characterized in that a measuring means which is arranged as near as possible to the treating location of a workpiece acts on the drive means of a conveyor mechanism by way of the control means in such a manner that the sand-shot quantity fed to the cleaning means does not exceed the capacity of the cleaning means.

16. A blasting apparatus according to claim 15, characterized in that the last-mentioned measuring means includes two induction coil means of which one is traversed by the shot mixture to be measured and the other is provided with a non-changing filling, the results of which are amplified and shaped into a control pulse in an amplifier means.

17. A blasting apparatus according to claim 16, characterized in that a heating means is provided for maintaining the two coil means at a predetermined temperature which corresponds approximately to the maximum shot temperature.

18. A blasting apparatus according to claim 17, characterized in that the induction coil means of the measuring means have each a length that is at least three times the inner diameter of a pipe means about which the two induction coil means are wound, said pipe means consisting of non-magnetic material with good heat insulation properties and a wall thickness of about one-third to one-fifth of the pipe diameter, and magnetic shield means about the coil means.

19. A blasting apparatus according to claim 18, characterized in that a heat insulation layer is arranged over the pipe means having the coil means wound thereon, the magnetic shield means being disposed about an outer heat-insulation.

20. A blasting apparatus according to claim 15, characterized in that the measuring means includes a source of radioactive rays and a ray-receiver means which are disposed on two sides of a line means conducting shot, the receiver means producing in its output the measurement result which is amplified and shaped into a pulse in an amplifier means.

21. A blasting apparatus according to claim 20, characterized in that the measuring means are arranged for determining the degree of contamination of the shot in a substantially vertically arranged line section which is followed downstream thereof by a throttle means.

22. A blasting apparatus according to claim 21, char acterized in that a metering means is provided for feeding fresh shot from a supply container to the circulatory system approximately to the same extent as use and loss thereof.

23. A blasting apparatus according to claim 22, characterized in that the blasting means is formed by centrifuging wheel means.

24. A blasting apparatus according to claim 1, characterized in that the measuring means are arranged for determining the degree of contamination of the shot in a substantially vertically arranged line section which is followed downstream thereof by a throttle means.

25. A blasting apparatus according to claim 1, characterized in that a metering means is provided for feed ing fresh shot from a supply container to the circulatory system approximately to the same extent as use and loss thereof. 

1. A blasting apparatus with a circulatory system for shot in which the contaminated shot traverses at least one cleaning means and in which the cleaned shot is fed to a blasting means, characterized in that at least one measuring means is provided in the circulatory system for determining the degree of contamination, said measuring means being operatively connected with a control means controlling the circulatory system by way of an actuating means and a mechanical control element in such a manner that at the inlet of the blasting means the shot does not fall below a predetermined degree of purity.
 2. A blasting apparatus according to claim 1, characterized in that the blasting means is formed by centrifuging wheel means.
 3. A blasting apparatus according to claim 1, characterized in that means are provided for feeding insufficiently cleaned shot back to the cleaning installation for renewed cleaning action.
 4. A blasting apparatus according to claim 1, characterized in that the measuring means is arranged downstream of and close to the treating place of a workpiece and in that the results of the measuring means act by way of the control means on the actuating means and the mechanical control element which is in the form of a throttle means, in such a manner that the shot supplied to the blasting means is so adjusted that the quantity of shot plus sand blasted off from the workpiece does not exceed the capacity of the cleaning means.
 5. A blasting apparatus according to claim 1, characterized in that a measuring means is arranged downstream of the cleaning means and upstream of the blasting means, the measuring results of said last-mentioned measuring means acting by way of the control means on the actuating means and mechanical control element which is in the form of a shifting valve means, in such a manner that sufficiently cleaned shot is fed to the blasting means and inadequately fed shot is fed back to the circulatory system for a renewed cleaning operation in by-passing relationship to the blasting means.
 6. A blasting apparatus according to claim 1, characterized in that a measuring means which is arranged as near as possible to the treating location of a workpiece acts on the drive means of a conveyor mechanism by way of the control means in such a manner that the sand-shot quantity fed to the cleaning means does not exceed the capacity of the cleaning means.
 7. A blasting apparatus according to claim 1, characterized in that the measuring means includes two induction coil means of which one is traversed by the shot mixture to be measured and the other is provided with a non-changing filling, the results of which are amplified and shaped into a control pulse in an amplifier means.
 8. A blasting apparatus according to claim 7, characterized in that a heating means is provided for maintaining the two coil means at a predetermined temperature which corresponds approximately to the maximum shot temperature.
 9. A blasting apparatus according to claim 8, characterized in that the induction coil means of the measuring means have each a length that is at least three times the inner diameter of a pipe means about which the two induction coil means are wound, said pipe means consisting of non-magnetic material with good heat insulation properties and a wall thickness of about one-third to one-fifth of the pipe diameter, and magnetic shield means about the coil means.
 10. A blasting apparatus according to claim 9, characterized in that a heat insulation layer is arranged over the pipe means having the coil means wound thereon, the magnetic shield means being disposed about an outer heat-insulation.
 11. A blasting apparatus according to claim 1, characterized in that the measuring means includes a source of radioactive rays and a ray-receiver means which are disposed on two sides of a line means conducting shot, the receiver means producing in its output the measurement result which is amplified and shaped into a pulse in an amplifier means.
 12. A blasting apparatus according to claim 11, characterized in that the measuring means are arranged for determining the degree of contamination of the shot in a substantially vertically arranged line section which is followed downstream thereof by a throttle means.
 13. A blasting apparatus according to claim 12, characterized in that a metering means is provided for feeding fresh shot from a supply container to the circulatory system approximately to the same extent as use and loss thereof.
 14. A blasting apparatus according to claim 4, characterized in that a measuring means is arranged downstream of the cleaning means and upstream of the blasting means, the measuring results of said last-mentioned measuring means acting by way of the control means on the actuating means and mechanical control element which is in the form of a shifting valve means, in such a manner that sufficiently cleaned shot is fed to the blasting means and inadequately fed shot is fed back to the circulatory system for a renewed cleaning operation in by-passing relationship to the blasting means.
 15. A blasting apparatus according to claim 14, characterized in that a measuring means which is arranged as near as possible to the treating location of a workpiece acts on the drive means of a conveyor mechanism by way of the control means in such a manner thaT the sand-shot quantity fed to the cleaning means does not exceed the capacity of the cleaning means.
 16. A blasting apparatus according to claim 15, characterized in that the last-mentioned measuring means includes two induction coil means of which one is traversed by the shot mixture to be measured and the other is provided with a non-changing filling, the results of which are amplified and shaped into a control pulse in an amplifier means.
 17. A blasting apparatus according to claim 16, characterized in that a heating means is provided for maintaining the two coil means at a predetermined temperature which corresponds approximately to the maximum shot temperature.
 18. A blasting apparatus according to claim 17, characterized in that the induction coil means of the measuring means have each a length that is at least three times the inner diameter of a pipe means about which the two induction coil means are wound, said pipe means consisting of non-magnetic material with good heat insulation properties and a wall thickness of about one-third to one-fifth of the pipe diameter, and magnetic shield means about the coil means.
 19. A blasting apparatus according to claim 18, characterized in that a heat insulation layer is arranged over the pipe means having the coil means wound thereon, the magnetic shield means being disposed about an outer heat-insulation.
 20. A blasting apparatus according to claim 15, characterized in that the measuring means includes a source of radioactive rays and a ray-receiver means which are disposed on two sides of a line means conducting shot, the receiver means producing in its output the measurement result which is amplified and shaped into a pulse in an amplifier means.
 21. A blasting apparatus according to claim 20, characterized in that the measuring means are arranged for determining the degree of contamination of the shot in a substantially vertically arranged line section which is followed downstream thereof by a throttle means.
 22. A blasting apparatus according to claim 21, characterized in that a metering means is provided for feeding fresh shot from a supply container to the circulatory system approximately to the same extent as use and loss thereof.
 23. A blasting apparatus according to claim 22, characterized in that the blasting means is formed by centrifuging wheel means.
 24. A blasting apparatus according to claim 1, characterized in that the measuring means are arranged for determining the degree of contamination of the shot in a substantially vertically arranged line section which is followed downstream thereof by a throttle means.
 25. A blasting apparatus according to claim 1, characterized in that a metering means is provided for feeding fresh shot from a supply container to the circulatory system approximately to the same extent as use and loss thereof. 